(a) Technical Field
The present invention relates to a pinhole inspection system and apparatus for a membrane electrode assembly (MEA) of a fuel cell, which detects the presence and position of a pinhole without damaging or destroying of one or more parts in the MEA.
(b) Background Art
A fuel cell is an energy conversion device that does not convert chemical energy of fuel into heat by combustion, but instead electrochemically converts the chemical energy directly into electrical energy. The fuel cell can then be used for the supply of electric power to small-sized electrical/electronic devices such as portable devices, as well as for the supply of electric power to industrial, domestic, and vehicle applications.
Typically, a polymer electrolyte membrane fuel cell (PEMFC) having high power density is conventional used as a fuel cell source in a vehicle. The polymer electrolyte membrane fuel cell has many advantages such as a low operating temperature of 50 to 100° C., fast startup and power conversion rate, and high energy density.
Structurally, a fuel cell in formed into a fuel cell stack by positioning a membrane electrode assembly (MEA) in the center of each unit cell of the fuel cell stack. The MEA comprises a solid polymer electrolyte membrane, through which hydrogen ions are transported, and catalyst layers including a cathode and an anode, which are coated on both sides of the electrolyte membrane so that hydrogen reacts with oxygen.
Moreover, a gas diffusion layer (GDL) and a gasket are sequentially stacked on the outside of the electrolyte membrane, i.e., where the cathode and the anode are positioned, respectively. A separator (also called a bipolar plate) including flow fields, through which reactant gases (hydrogen as a fuel and oxygen or air as an oxidant) are supplied and coolant passes therethrough, is positioned on the outside of the GDL. Then, a plurality of unit cells are stacked, and an end plate for supporting the unit cells is attached to each of the outermost sides so that the unit cells can be arranged and fastened between the end plates, thus constructing a fuel cell stack.
As one can image, deterioration of the polymer electrolyte membrane, a core component of the fuel cell, has significant effects on the degradation in performance of the PEMFC. In particular, the deterioration of the electrolyte membrane occurs via a mechanism represented by the following Reaction s (1) to (3).R—CF2COOH+HO.→R—CF2+CO2+H2O  Reaction (1):R—CF2+HO.→R—CF2OH→R—COF+HF  Reaction (2):R—COF+H2O→R—COOH+HF  Reaction (3):
Referring to the above Reaction s, oxygen diffusing through the membrane from the cathode is converted into hydrogen peroxide in the presence of a platinum (Pt) catalyst and diffuses to the electrolyte membrane. Here, when the membrane is contaminated by metal ions, hydroxyl radicals (HO.) and hydroperoxyl radicals (HOO.) are produced by a reaction between hydrogen peroxide and metal ions. These radicals attack the end groups of the membrane to cause chain scission in the polymer chains of the membrane, thus deteriorating the polymer electrolyte membrane.
When the scission of the polymer chains is repeated by the above chemical deterioration, the elastomeric polymer may become thin, or local pinholes or cracks, etc., may occur. These deterioration effects are associated with the degradation in performance of the fuel cell stack. Accordingly, it is necessary to detect the presence and position of the pinhole in the polymer electrolyte membrane in order to prevent the degradation in performance of the fuel cell stack.
Conventional techniques to detect the presence and position of the pinhole in the polymer electrolyte membrane will now be described.
First, as shown in FIG. 1, Japanese Patent Publication No. 2004-233097 discloses a hydrogen sensor 5 that includes a microcapsule means 1 for covering the powder particles of a hydrogen-occlusion alloy with a metal film, a temperature detection means by a thermocouple 2, an integration means, where the covered powder particles of the hydrogen occlusion alloy of the microcapsule means 1 and the thermocouple 2 of the temperature detection means are accommodated in an end cap of a fuse 3, and an electronic control means by an electronic control unit 4 including a power supply.
However, when the pinhole of the membrane electrolyte assembly is inspected using the hydrogen sensor disclosed in the above Japanese Patent Publication document, only the amount (concentration) of hydrogen gas passing through the pinhole can be detected. It is not possible to measure the position or size of the pinhole.
Secondly, FIG. 2 is a schematic diagram showing a conventional method for detecting a pinhole in a polymer electrolyte membrane using ammonia, in which diluted ammonia (NH3) gas is applied to one side of a polymer electrolyte membrane 11 through a gas flow field 12, and a pH indicator 13 (Filter Paper with Congo Red) is installed on the other side, thus detecting the pinhole in the polymer electrolyte membrane 11 via an Ammonia (NH3) Gas Method as disclosed in Shinji Kinoshita, Fuel Cell Testing Workshop 2007.
However, ammonia gas is very irritating to the eyes, nose, and throat and, the exposure to ammonia gas for a long period of time may cause breathing difficulties, which is very harmful to humans. Accordingly, when the inspection of the pinhole is performed by the above method, it is impossible to perform a total inspection for a long period of time and reuse the polymer electrolyte membrane electrode assembly after the inspection.
Third, as shown in FIG. 3, Korean Patent Application Publication No. 10-2009-0107610, filed by the present applicant, which is hereby incorporated in its entirety by reference, illustrates an apparatus and method for detecting the position of a pinhole in a polymer electrolyte membrane. In particular, an upper block 20 and a lower block 21, in which solutions having different pH values are stored, respectively. An indicator inlet 22 penetrates the top of the upper block 20, and a pair of intermediate plates 23a and 23b are disposed between the upper block 20 and the lower block 21. A lattice-type support net 25 is attached to a support net installation area 24 in the middle of the pair of intermediate plates 23a and 23b, and a polymer electrolyte membrane 25 whose edge is fixed between the pair of intermediate plates 23a and 23b and whose center is placed on the support net, so that the solutions having different pH values can be brought into contact with both sides of the polymer electrolyte membrane 27. Then an indicator 26 is injected into the one side of the polymer electrolyte membrane 25 to cause deterioration, thus easily detecting the position of a pinhole formed in the polymer electrolyte membrane 25 by the deterioration.
During the detection of the presence of the pinhole in the polymer electrolyte membrane 25 using the above method, the polymer electrolyte membrane 25 is exposed to acidic or alkaline pH solutions and thus cannot be reused after the inspection, and the waste solutions are difficult to treat and cause environmental pollution. Therefore, although effective are not ideal.
Lastly, FIG. 4 is a schematic diagram showing a conventional method and apparatus for detecting the presence of a pinhole 31 in a polymer electrolyte membrane using an infrared camera 30 as is disclosed in U.S. Pat. No. 5,763,765. In this patent, a polymer electrolyte membrane electrode assembly is fixed to a fixture, air is injected into one side thereof, and hydrogen is injected into the other side thereof. If there is a pinhole in the polymer electrolyte membrane electrode assembly, the two gases meet through the pinhole to cause an exothermic reaction in the presence of a platinum (Pt) catalyst. Here, the temperature distribution is measured by an infrared camera 30, and thus the point at which the temperature increases locally is determined as the position of the pinhole 31. The numeral 32 denotes a pinhole detection monitor, and 33 denotes an electrical signal analyzer.
However, when an infrared camera 30 is used, hydrogen gas is injected into a confined space and then pressurized, and thus the membrane may be torn by this process. Moreover, when the exothermic reaction between hydrogen and air occurs suddenly at high temperature during the pressurization, the polymer electrolyte membrane may be burned as well, thus rendering it unusable.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.